1. Field of the Invention
This invention generally relates to communication receivers and, more particularly, to a system and method for adaptively controlling the gain switch points in a wireless communications receiver.
2. Description of the Related Art
FIG. 8 is a schematic block diagram illustrating a conventional wireless communications receiver (prior art). Ideally, a wireless communications device receives desired, inband communication signals at appropriate power levels. However, in practice, received signals often include undesired inband signals, referred to as jammers. Further, the receiver must have an adjustable gain to create appropriate power level signals for subsequent processing and demodulation. Jammers and inband communication signals with high power levels can cause distortion and intermodulation products when amplified. The linearity of, and conversely, the distortion in the receiver can be measured by the third order intercept point (IP3). Generally speaking, the higher the IP3, the more linear the amplifier operation. Alternately stated, the IP3 and linearity are improved by reducing gain through the amplifier.
On the other hand, a low power received signal must be amplified with a minimum of loss prior to the low noise amplifier (LNA) stage. Unfortunately, the gain reductions that are performed to minimize the creation of intermodulation products associated with jammers can work against the selection of optimal gain and noise figure needed to recover low power received signals.
Conventionally, a receiver LNA, attenuator, or mixer is set to operate in a series of predetermined gain states. Using the LNA of FIG. 8 as an example, the LNA may have four different gain states that are triggered solely in response to the total inband energy or power measured by the automatic gain control (AGC) circuit. For example, the LNA may be set to operate in a first gain state with a gain of 20 db, and a noise figure of 3 dB, in response to the AGC measuring a power level of −90 dBm or less. When the AGC measures greater than −90 dBm and less than −60 dBm for example, the LNA may be set to set to operate in a second gain state with a gain of 15 dB and a noise figure of 5 dB. In the above example, the LNA set points are −90 dBm and −60 dBm. The gain states are chosen in consideration of C/N or optimum receiver performance.
FIG. 9 is a graph showing the ratio of desired signal energy and total signal energy, or carrier to noise (C/N) ratio, plotted against input power (PIN) (prior art). Using code division multiplexed access (CDMA) telephone voice communications as an example, to hold frame error rate to 0.5% or less per 1,000 frames, the C/N of a receive signal must be kept above the −1 dB line shown in FIG. 9. The sudden spikes of degradation in C/N are associated with the above-mentioned LNA switch points.
Wide bandwidth telephone data applications are being developed, such as Internet access and video communications. Generally, the higher the C/N, the faster the wideband applications can be communicated. For example, in some circumstances, a 3 dB increase in the C/N doubles digital data carrying capacity.
As noted above, the trade off made between linearity and noise figure may limit receiver performance. This limitation may be acceptable in circumstances where only a minimum C/N is required, such as in conventional voice traffic. However, these limitations often prevent a receiver from being set to optimally process an intended signal. Alternately stated, a convention receiver design does not optimize the C/N ratio, as would be helpful in the communication of wideband data applications.
It would be advantageous if receiver performance could be adaptively modified to optimize the C/N for greater data communication capacity.
It would be advantageous if receiver performance could be controlled to optimize the discernable signal of interest of a signal with respect to the total inband power.
It would be advantageous if the receiver amplification set points be modified to adapt to changing receiving scenarios.